As further miniaturization is pursued with increasing vigor in the field of semiconductor production and semiconductor chip substrate production today, the projection optical system in an exposure apparatus that prints patterns needs to achieve higher resolution. The wavelength of the exposure light must be reduced and the NA (numerical aperture at the projection optical system) must be increased to raise the resolution. However, since light absorption becomes a factor that needs to be taken into consideration, only limited types of optical glass can be utilized in practical application in conjunction with exposure light with small wavelengths. For instance, if the wavelength is 180 nm or less, only fluor can be utilized as glass material in practical application.
In such a situation, if the projection optical system is constituted by using refractive optical members (lenses, plane parallel plates, etc.) alone, it is impossible to correct any chromatic aberration with the refractive projection optical system. In other words, it is extremely difficult to constitute a projection optical system achieving the required resolution with refractive optical members alone. In response, attempts have been made to constitute a projection optical system with reflective optical members, i.e., reflecting mirrors, alone.
However, the reflective projection optical system achieved by using reflective optical members alone is bound to become large. In addition, aspherical reflecting surfaces must be formed. It is to be noted that it is extremely difficult to form a high precision aspherical reflecting surface in the actual manufacturing process. Accordingly, various so-called catadioptric reducing optical systems achieved by using refractive optical members constituted of both optical glass that can be used in conjunction with exposure light with small wavelengths and reflecting mirrors, have been proposed.
Among these catadioptric systems, there is a type of catadioptric system that forms an intermediate image only once by using a single concave reflecting mirror. In such a catadioptric system, the optical system portion for reciprocal paths, of which the concave reflecting mirror is a component, only includes negative lenses and does not have any refractive optical member with positive power. As a result, light enters the concave reflecting mirror as a wide light flux, which requires that the concave reflecting mirror have a large diameter.
In particular, when the optical system portion for reciprocal paths having the concave reflecting mirror adopts a completely symmetrical configuration, the onus of the aberration correction imposed on the refractive optical system portion at the succeeding stage is reduced by minimizing the occurrence of aberration at the optical system portion for reciprocal paths. However, a sufficient working distance cannot be readily assured in the vicinity of the first plane in the symmetrical optical system for reciprocal paths. In addition, a half prism must be used to branch the optical path.
Furthermore, if a concave reflecting mirror is used at a secondary image forming optical system provided to the rear of the position at which the intermediate image is formed, the light needs to enter the concave reflecting mirror as a wide light flux in order to assure the degree of brightness required by the optical system. As a result, it is difficult to miniaturize the concave reflecting mirror whose diameter tends to be necessarily large.
There is also a type of catadioptric system that forms an intermediate image only once by using a plurality of reflecting mirrors. In this type of catadioptric optical system, the number of lenses required to constitute the refractive optical system portion can be reduced. However, the following problems must be addressed with regard to such catadioptric systems.
In the catadioptric system having the optical system portion for reciprocal paths adopting the structure described above provided toward the second plane on the reduction side, other restrictions imposed with regard to the reduction factor make it impossible to assure a sufficiently long distance to the second plane (the wafer surface) over which the light travels after it is reflected at the reflecting mirror. For this reason, a large number of lenses cannot be inserted in this optical path and the level of brightness achieved at the optical system is bound to be limited. In addition, even if an optical system with a large numerical aperture is achieved, numerous refractive optical members must be provided in the optical path with a limited length, and thus, it is not possible to assure a sufficiently long distance between the wafer surface at the second plane and the surface of the lens toward the second plane, i.e., the so-called working distance WD cannot be set to a sufficiently long value.
Since the optical path has to be bent, the catadioptric system in the related art is bound to have a plurality of optical axes (an optical axis is a line extending through the center of the curvature of a refractive curved surface or a reflective curved surface constituting an optical system). As a result, a plurality of lens barrels must be included in the optical system configuration, which makes it extremely difficult to adjust the individual optical axes relative to one another and, ultimately, to achieve a high precision optical system. It is to be noted that a catadioptric system having all the optical members provided along a single linear optical axis may be achieved by using a pair of reflecting mirrors each having an opening (a light transmission portion) at the center. However, in this type of catadioptric system, the central light flux must be blocked, i.e., central shielding must be achieved, in order to block off unnecessary light which advances along the optical axis without being reflected at the reflecting mirrors. As a result, a problem arises in that the contrast becomes lowered in a specific frequency pattern due to the central shielding.
In addition, positions at which an effective field stop and an effective aperture stop should be installed can not be assured in the catadioptric systems in the related art. Also, as explained above, a sufficient working distance cannot be assured in the catadioptric systems in the related art. Furthermore concave reflecting mirrors tend to become large in the catadioptric systems in the related art, as described above, making it impossible to miniaturize the optical systems.
While the catadioptric system disclosed in EP1069448A1 achieves advantages in that a sufficient working distance is assured toward the second plane (on the wafer-side) and in that the system is configured along a single optical axis, it still has a problem in that a sufficiently long working distance cannot be assured toward the first plane (on the mask side) (the distance between the mask surface at the first plane and the surface of the lens closest to the first plane). In addition, the catadioptric system disclosed in WO 01/51979A2 poses a problem in that the diameter of the reflecting mirror is too large and thus, a sufficiently large numerical aperture cannot be achieved. Likewise, the diameter of the reflecting mirror is too large and, as a result, a sufficiently large numerical aperture cannot be achieved in the catadioptric system disclosed in Japanese Laid-open Patent Publication No. 2001-228401.